Delivery (transport) fibers are commonly used to transport laser light (radiation) from a source thereof to a point of usage. This technique allows for a convenient separation of the source from the point of usage by many meters. State-of-the-art fiber delivery arrangements are able to transport continuous-wave (CW) laser radiation with powers of up to tens of kilowatts (kW) over distances of up to hundreds of meters. Such delivery arrangements typically employ a transport fiber having solid glass core surrounded by claddings and jackets to guide the radiation and protect the fiber.
When used with ultra-short pulsed, high-energy lasers, this solid glass core decreases the quality of the pulse in temporal and spectral domains due to nonlinear effects in the glass. This can lead to problems including an increased pulse-duration, and a severely distorted temporal pulse-profile (pulse-shape). In an extreme case of very high peak-power, for example about 5 megawatts (MW) or greater, the solid glass core of the delivery fiber can be destroyed.
A known solution to the problem is to substitute a hollow-core fiber (HCF) for the solid-glass-core fiber. A hollow-core fiber is a fiber in which radiation propagates primarily in a central hollow region surrounded by cladding material typically referred to as photonic crystal or photonic bandgap material. The photonic crystal material is surrounded by solid cladding material. Photonic crystal material is a mixture of solid (glass) and void regions (longitudinally-extending tubes) arranged in a particular pattern. Hollow-core fibers are commercially available from a number of suppliers and include types referred to as photonic bandgap fibers, Kagome lattice fibers, and anti-resonant fibers. FIG. 1, FIG. 2, and FIG. 3 are a cross-section micrographs schematically illustrating, respectively, examples of these three hollow-core fiber types.
In an HCF, the laser-radiation propagates primarily in air, some other gas, or vacuum, with only a small portion of radiation light propagating in glass. Because of this, the above-discussed nonlinear effects can be greatly decreased, and a high pulse-quality is maintained throughout the propagation in the fiber. This enables the transport of high energy picosecond (ps) and femtosecond (fs) pulses through the fiber with only minimal change to pulse-duration and pulse-shape.
In certain applications of pulsed laser-radiation, the radiation is delivered from a laser nominally plane-polarized in a preferred orientation, and it is desired that this polarization state is maintained at the point of usage after being transported thereto by a transport fiber. A particular challenge in the use of an HCF for laser-radiation transport is preserving (maintaining) the plane polarization of the laser-radiation during transport. It is possible to maintain the polarization orientation throughout the transport by carefully matching preferred polarization orientations of the HCF. Unfortunately, these orientations can rotate and change during operation, making realignment of the radiation-source and the HCF necessary. Parameters influencing the polarization-orientation include fiber temperature, temperature gradient, and fiber bending. Fiber bending limits substantially the use of an HCF for transporting plane-polarized radiation.
Moving the HCF or changing bending-planes will rotate and change the polarization state of the laser-radiation. In order to take advantage of an HCF for above described low-distortion transport of high-energy radiation pulses, a means is required for preserving plane-polarized radiation at an output of the fiber.